The invention relates to the field of using a liquid reaction medium for culturing microorganisms or isolated cells of vegetable or animal macroorganisms.
More precisely, the invention relates to a method of improving the yield of a biological reactor comprising at least one annular biological reaction chamber defined by two coaxial cylindrical walls, at least one of which is an exchange wall allowing gaseous or liquid matter to be transferred or allowing light to pass, and in which there flows axially a liquid reaction medium containing a culture of microorganisms or of isolated cells of vegetable or animal microorganisms in suspension, in which method the culture is subjected to a biosynthesis reaction activated by the matter or the light passing through said exchange wall.
The invention relates both to culturing phototrophic microorganisms or isolated cells, and to culturing heterotrophic microorganisms or isolated cells.
When culturing phototrophic microorganisms, visible light radiation and carbon dioxide gas are supplied, and the gas is consumed during the photosynthesis reaction with oxygen being produced. The oxygen must be removed since it is toxic above a certain concentration.
When culturing heterotrophic microorganisms, oxygen is supplied. The oxygen is consumed and carbon dioxide gas is produced which must be eliminated since it is toxic above a certain concentration.
FR A 2 762 326 describes a photobioreactor for culturing photoautotrophic microorganisms, the reactor comprising a plurality of horizontal annular chambers having a transparent inside wall surrounding a light chamber. The reaction medium is introduced axially into the annular chamber where it is subjected to turbulent movement.
When a liquid is introduced axially into an annular chamber, the molecules always remain at substantially the same distance from the axis of the chamber. If the movement is turbulent, then the molecules are moved short distances radially about a mean position. It can be deduced that the culture situated in the vicinity of the inside wall is strongly illuminated, whereas the culture located close to the outside wall is poorly illuminated. Very strong turbulence might perhaps lead to the culture in the vicinity of the inside wall being renewed. However microorganisms are sensitive to shear, and high levels of turbulence are harmful for production.
In FR A 2 762 326, the absorbance of the culture is controlled by adding transparent or reflecting particles to the reaction medium, said particles being of a density that is substantially equal to that of the reaction medium, with the volume percentage of the particles in the reaction medium being a function of the species of microorganism being cultivated, of the thickness of the chamber, and of the desired final concentration of the culture in the reaction medium.
That disposition makes it possible to ensure that all of the microorganisms are illuminated throughout their transfer in the annular chamber, and regardless of the type of microorganism. However, it does not enable growth to be continued in darkness after the microorganisms have been illuminated for a duration Tlum.
The object of the invention is to improve the yield of that type of photobioreactor by creating a flow in the chamber that encourages renewal of the culture in the vicinity of the illuminated wall so that microorganisms that have been illuminated continue their growth when they enter into non-illuminated zones.
Thus, for a given flow rate of the reaction medium, microorganism growth is greater than that obtained by the known method.
The method of the invention is equally applicable to isolated cells or microorganisms other than phototrophic microorganisms.
In particular, when the biological reaction is obtained by consuming a gas (carbon dioxide gas or oxygen), and producing another gas for elimination (oxygen or carbon dioxide gas), the inside wall of the biological reaction chamber may be constituted by a hydrophobic semipermeable membrane which retains the reaction medium inside the chamber while allowing one of the gases to pass through. When feeding a gas, the gas is under pressure in the inside tube of the chamber (source), and eliminating a gas, the inside chamber (well) is under suction.
It should be observed that the gas exchange wall may be the outside wall of the biological reaction chamber, in which case the chamber is placed in a duct of larger diameter than is used for delivering a gas, preferably under pressure (source). Both walls of the biological reaction chamber may be semipermeable and hydrophobic, with the inside wall forming a well under suction for eliminating the toxic gas and with the outside wall being connected to a source of the gas to be consumed during the biological reaction. Similarly, one of the walls may be transparent to enable the culture to be illuminated while the other wall may be semipermeable and preferably hydrophobic to enable a gas to be delivered or a toxic gas to be eliminated after being produced during the photobioreaction.
When the biological reaction requires an organic or inorganic nutrient solution to be supplied, the inside wall of the biological reaction chamber is constituted by a hydrophilic porous membrane and defines a source of nutrient solution under pressure.
The method of the invention is characterized by the fact that the reaction medium flowing in said biological reaction chamber is subjected to a primary turbulent flow that is helical about the axis of the chamber, which flow under the action of centrifugal force generates rotary secondary vortices, thereby encouraging the culture to be renewed in the vicinity of the exchange wall.
Advantageously, the reaction medium is subjected to a helical turbulent primary flow by introducing the reaction medium at one of the ends of the biological reaction chamber in a direction that is substantially perpendicular to the axis of the chamber and that is offset away from said axis.
Preferably, the reaction medium is introduced into the biological reaction chamber by means of a duct having an axis that is substantially perpendicular to the axis of the chamber, and that is connected tangentially to the outer wall of said chamber.
The internal transverse dimension of the duct in the direction perpendicular to the axis of the biological reaction chamber is no greater than the radial thickness of said chamber.